Semiconductor devices (e.g., memory devices, processor devices, light-emitting diodes (LEDs)), as well as micro-electromechanical system (MEMS) devices often include repeating features that are formed in a pattern across a portion of the devices, such as in arrays. For example, some semiconductor devices include an array of transistors with associated features, such as capacitors, electrically conductive lines and vias, electrically conductive contacts, etc. As device features are reduced in size, conventional processing techniques (e.g., photolithography) are unable to directly meet the size requirements.
The concept of pitch may be used to describe the sizes of features of a semiconductor device. Pitch is defined as a distance between an identical point in two adjacent features when the pattern includes repeating features. These features are conventionally separated by spaces that are filled by a material, such as an insulator. Pitch can be viewed as the sum of the width of a feature and of the width of the space on one side of the feature separating that feature from an adjacent feature. To increase the capacity of the semiconductor devices of a given size, features are formed at an increased density (i.e., increased number of features per area). Accordingly, each feature is formed at a size and pitch to reliably fit a desired number of the features in a given area at a reasonable cost.
Photolithography is a technique used to form repeating features in semiconductor devices. Generally, photolithography is performed by forming a photosensitive material (e.g., a photoresist) over another material. Using a so-called “positive tone” photosensitive material, radiation of an appropriate wavelength is directed onto portions of the photosensitive material that are to be removed. The radiation chemically alters the photosensitive material to enable the photosensitive material to be soluble in and removed by a solution (e.g., a developer solution), while portions of the photosensitive material that have not been exposed to the radiation remain insoluble in the solution and are not removed by the solution. Material underlying the photosensitive material is removed through the openings formed by removal of the exposed portions of the photosensitive material and/or formed within the openings to form the features in a desired pattern. A so-called “negative tone” photosensitive material functions similarly, except that the portion thereof exposed to the appropriate wavelength of radiation becomes insoluble in the solution, while portions of the photosensitive material that have not been exposed to the radiation remain soluble in and removable with the solution.
However, due to factors such as optics limitations and usable radiation wavelengths, photolithography techniques have a minimum pitch below which a particular photolithographic technique cannot reliably form features. Thus, the minimum pitch of a photolithographic technique is an obstacle to continued feature size reduction to smaller critical dimensions. “Critical dimension,” as used herein, means and includes the smallest dimension of a feature of a structure or recess (e.g., contact, line, trench, etc.) of a semiconductor device structure. “Pitch multiplication” or “pitch doubling” is a process that has been used to form features smaller than is reliably possible by conventional photolithography techniques. While pitch is actually reduced by this technique, the reduction in pitch is conventionally referred to as “pitch doubling” or, more generally, “pitch multiplication.” Thus, conventionally, “multiplication” of pitch by a certain factor actually involves reducing the pitch by that factor. The conventional terminology is retained herein. Currently, the smallest critical dimension obtainable in, for example, lines and spaces between the lines of a semiconductor device structure using conventional 193 nm photolithographic techniques and/or so-called “litho-litho-etch” techniques known to the inventors herein is 37.5 nm.
In one method of pitch doubling, a pattern of photosensitive material is formed by conventional photolithography and a spacer is formed on sidewalls of the photosensitive material. Material from the spacer is removed from horizontal surfaces (e.g., a top of the photosensitive material, a floor of a space between adjacent portions of the pattern), leaving the spacers only along the sidewalls of the photosensitive material. The photosensitive material is removed, leaving two spacers for every one portion of photosensitive material originally formed by photolithography (e.g., one spacer on each of two opposing sidewalls). The spacers form a pattern, which is transferred into an underlying material. Material underlying the spacers is retained, while material underlying an area between the spacers is removed to form features in the underlying material in a desired pattern. Alternatively or additionally, material may be formed (e.g., deposited) between the spacers, between features underlying the spacers, or within openings and trenches formed under the spacers. Thus, a number of features can essentially be doubled in a given area, compared to conventional photolithography techniques.
However, pitch doubling techniques involve an undesirable number of process acts to arrive at a final pattern and conventional techniques limit the smallest critical dimension. Alternative, improved methods for fabricating features of dimensions below resolution limits of photolithography are desirable.